Help: Document cmake-presets version 8

This was accidentally missed in commit f1a7179520 (presets: Add $schema
property to JSON schema, 2023-09-07, v3.28.0-rc1~106^2).

Help/manual/cmake-presets.7.rst

@@ -75,6 +75,9 @@ The root object recognizes the following fields:
   ``7``
     .. versionadded:: 3.27
 
+  ``8``
+    .. versionadded:: 3.28
+
 ``cmakeMinimumRequired``
   An optional object representing the minimum version of CMake needed to
   build this project. This object consists of the following fields: